Instrument & cuvette calibration.

The calibration of the instrument is a step required at least once every two months, but it should be performed also at least once with each short path-length cuvette (< 0.5 cm). Once you knew the calibration of a given cuvette, you will not need to do it anymore (but you still have to check the instrument at least every two months).
The usual standard for the calibration is d-10-Camphorsulfonic Acid in water, whose CD spectrum provides a two-point calibration: a negative CD band at 192.5 nm and a positive band at 290.5 nm. The ratio of the CD intensities at these wavelengths should be around -2.00 (from -2.40 to -2.00), but the aging of the Xenon lamp (or some problem in the instrument setting!) can drop it down to -1.90.
Please refer to table 1 and to reference figure I for the d-Camphorsulfonic acid spectral characteristics.
After calibrating the instrument with a 1cm cuvette, if you want to use a cuvette with an inferior path length you should calibrate that particular cuvette (if you never did it before). This means to measure the CD of same d-camphorsulfonic acid solution in the cuvette to calibrate and to verify that a reduction of the CD signal proportional to the new path length is observed. For example, if you use a cuvette whose nominal path length is 0.5 cm, you should observe a reduction of the intensity of the CD spectrum to one half of the 1cm cuvette intensity.
Any deviation from the predicted rule should be due to path length inaccuracy and if superior to 1% should be considered when normalizing the data at the end of the experiment.

"Rough spectrum" acquisition.

Now you are ready for a first trial.
Before recording the actual spectrum, it is better to do a coarse measurement to ensure that the protein concentration is OK and that the spectrum itself has a "good" appearance.
To do so, put the sample in your instrument and use the following settings:

Wavelength range : 180-260 nm (at least 185-260 nm)
Scan speed: 100 nm/min
Response: 2sec
Accumulation: 1
Spectral width: 1nm
Resolution: 1nm

Before starting, activate the potential signal window (View/Channel 2 for Jasco users).

At the end of data acquisition (less than 1 min!) you should have a quite good spectrum with a maximum in the range of 10-20 mdeg. (absolute values) and a voltage signal never bigger than 700. If the signal is too intense or too low you MUST adjust the protein concentration to the correct value and repeat the spectrum acquisition.